1. Field of the Invention
The present invention relates generally to signal output circuits, and more particularly to a signal output circuit having the function of shutting down the operation of an internal circuit (a shutdown function) and the function of muting a signal (a mute function).
2. Description of the Related Art
A conventional audio amplifier circuit that amplifies an audio signal and outputs the audio signal to headphones or a speaker is known.
Such an audio amplifier circuit has a shutdown function and a mute function for cutting noise at the time of turning on or turning off power.
FIG. 1 is a block diagram showing a conventional audio amplifier circuit 101.
An input signal is supplied from a signal source 102 to the input terminal Tin of the audio amplifier circuit 101 via a capacitor C41 for cutting a direct current. The input signal supplied to the input terminal Tin is supplied to an amplifier circuit 111. The amplifier circuit 111 includes a differential amplifier circuit 121, an input resistor R31, a feedback resistor R32, and a switch 122, and has a reference voltage applied thereto from a reference voltage generator circuit 112, thus forming an inverting amplifier circuit.
The amplifier circuit 111 outputs a signal that is a function of the difference between the reference voltage from the reference voltage generator circuit 112 and the input signal supplied to the input terminal Tin. The signal amplified by the amplifier circuit 111 is output from an output terminal Tout to drive a speaker 103.
The switch 122 is provided between the connection point of the input resistor R31 and the feedback resistor R32 and the inverting input terminal of the differential amplifier circuit 121. The switch 122 is switched in accordance with a mute signal supplied from a controller 104 to a control terminal Tcnt1. When the level of the mute signal is HIGH, the switch 122 short-circuits the connection point of the input resistor R31 and the feedback resistor R32 and the inverting input terminal of the differential amplifier circuit 121 so that the input signal is inverted and amplified to be output from the output terminal Tout.
When the level of the mute signal is LOW, the switch 122 short-circuits the output of the differential amplifier circuit 121 and its inverting input terminal, thereby causing a mute state so as to prevent the input signal from being output from the output terminal Tout. Thus, the switch 122 is switched in accordance with the mute signal supplied from the controller 104 to the control terminal Tcnt1, so that the supply of the input signal to the differential amplifier circuit 121 is controlled. As a result, the mute function is controlled.
The reference voltage generator circuit 112 includes a switch 131, resistors R41 and R42, and a capacitor C51. A constant voltage Vdd is applied to the reference voltage generator circuit 112. The constant voltage Vdd is applied via the switch 131 to a series circuit formed of the resistors R41 and R42. When the level of a shutdown signal supplied from the controller 104 to a control terminal Tcnt2 is HIGH, the switch 131 is turned ON to apply the constant voltage Vdd to the series circuit formed of the resistors R41 and R42. When the level of the shutdown signal is LOW, the switch 131 is turned OFF to stop the applying of the constant voltage Vdd to the series circuit formed of the resistors R41 and R42.
When the switch 131 is ON, the resistors R41 and R42 divide the constant voltage Vdd to generate the reference voltage. The generated reference voltage is supplied to the non-inverting input terminal of the differential amplifier circuit 121. As a result, the amplifier circuit 111 is put in an operating state. A terminal Tc, is connected to the connection point of the resistors R41 and R42, and an external capacitor C51 is connected to the terminal Tc. The capacitor C51 connected to the terminal Tc absorbs the ripple of the reference voltage to stabilize the reference voltage.
Next, a description is given of the operation of the audio amplifier circuit 101.
FIG. 2 is a diagram for illustrating the operation of the audio amplifier circuit 101. In FIG. 2, (a) indicates the shutdown signal output from the controller 104, (b) indicates the switching of the switch 131, (c) indicates the reference voltage supplied to the differential amplifier circuit 121, (d) indicates the mute signal output from the controller 104, and (e) indicates the switching of the switch 122.
Referring to FIG. 2, when the level of the shutdown signal switches from LOW to HIGH at a time t10 as shown in (a), the switch 131 is switched from OFF state to ON state as shown in (b). As a result of the switching of the switch 131 to ON state, the reference voltage is generated by the resistors R41 and R42. At this point, as shown in (c), the reference voltage is caused by the external capacitor C51 to gradually rise to reach a predetermined level at a time t11. When the reference voltage reaches the predetermined level at the time t11, the differential amplifier circuit 121 is released from a shutdown state to enter an operating state.
The controller 104 counts a predetermined period of time since the time t10, when the level of the shutdown signal switches to HIGH. The controller 104 outputs the mute signal at a time t12, when the preset predetermined period of time passes, as shown in (d) of FIG. 2. The mute signal turns ON the switch 122 of the amplifier circuit 111 as shown in (e) of FIG. 2. As a result, the input signal is released from a mute state, and is amplified by the amplifier circuit 111 to be supplied to the speaker 103.
Thus, according to the conventional audio amplifier circuit 101, the generation of the reference voltage in the reference voltage generator circuit 112 is controlled based on the shutdown signal supplied from the controller 104. As a result, the operation of the amplifier circuit 111 is controlled, and thus, the shutdown function is controlled. Further, the mute function of the amplifier circuit 111 is controlled based on the mute signal supplied from the controller 104.
According to the conventional audio amplifier circuit, the shutdown signal and the mute signal should be input separately to an integrated circuit. This increases the number of external pins of the integrated circuit, thus making it difficult to reduce the size of the integrated circuit.
Therefore, in order to reduce the number of external pins, an audio amplifier circuit that controls both the shutdown function and the mute function in accordance with the level of the shutdown signal has been proposed (U.S. Pat. No. 5,642,074).
According to the conventional audio amplifier circuit where the shutdown signal for controlling the shutdown function and the mute signal for controlling the mute function are separately supplied from an external controller and control the shutdown function and the mute function, respectively, it is necessary to generate the shutdown signal and the mute signal in the external controller, and it is necessary to control timing for generating these signals. This causes a problem in that a load is imposed on the processing of the controller.
According to the conventional audio amplifier circuit controlling both of the shutdown function and the mute function in accordance with the level of the shutdown signal, timing for controlling the shutdown function and timing for controlling the mute function cannot be determined with accuracy, so that in some cases, noise may be output when an amplifier is activated.